This invention in general relates to a system for precisely positioning a writing element along a rotating drum surface to, for example, print on a medium held by the drum and, more particularly, to such a system incorporating a linear grid disposed along a path of motion and a set of markings of varying density fixed to the drum for developing phase-locked signals for aligning the writing element position with drum position.
A common form of system in which a writing element is driven along a drum surface is found in a printer in which a rotating drum carries a printing medium past a stylus. The drum has a cylindrical surface and rotates about an axis of the drum. The stylus is carried by a carriage which is driven in a direction parallel to the drum axis by a lead screw. Electronic circuitry, which may include a memory for storing image data in the form of an array of pixels, actuates the stylus to print marks on the medium as the drum and the lead screw rotate.
In constructing such a printer, the leadscrew must be configured accurately to insure that the stylus translates precisely along a path parallel to the drum axis as the drum rotates since pixel data to be imprinted is addressed in the memory on the basis of the position which the pixel will have on the spiral. Any lack of precision in the construction of the lead screw, such as backlash between screw and carriage, may introduce a slight offset in pixel data from the prescribed position along the spiral. This results in a reduction of the accuracy of the construction of the image on the printing medium. Accordingly, it has been necessary to employ considerable care and expense in the construction of the lead screw and the carriage.
While the foregoing problem has been noticed in the case of printers employing a lead screw for translation of a stylus, it is to be understood that other mechanisms for the translation of the stylus may also require excessive care and expense in their construction. Furthermore, the problem is not restricted only to printers, but may also be found in other situations employing apparatus wherein a stylus or arm is to be translated along a path parallel to an axis of rotation of a drum wherein a tip of the arm is to interact with the drum at precise locations along the surface of the drum.